<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    .qq{
        color: blue;
        list-style: none;
        text-decoration: underline;
    }
  
</style>
<body>
    <button>我是按钮</button>
    <ul>
        <li>问天</li>
        <li>问地</li>
        <li>问星河</li>
    </ul>
</body>
</html>
<script>
 
    var an=document.querySelector("button");
    an.onclick=function(){
        var li=document.querySelectorAll('li');
      for(i=0;i<li.length;i++){
       li[i].className='qq';
      }
 
    }
</script>